The neural substrates of emotional response have traditionally been studied using universal sets of emotionally loaded stimuli, regardless of their subjective significance to the individual subject. Assessment of the unique brain response to subjectively significant stimuli has not been studied before.
Deception is probably the best-studied mind states of all; most of the present lie-detection testing techniques use polygraph devices to examine the peripheral autonomic response to relevant versus irrelevant questions. Present day polygraph devices can combine measurements of electro-dermal skin conductance, blood pressure, respiration and peripheral vasomotor activity, minute changes in vocal response and face temperature. The increase in autonomic response is interpreted as an attempt to deceive by the investigated subject. This basic principle behind polygraph machine hasn't changed since its invention over 80 years ago.
Lately, a newer method for lie detection which is based on examining the amplitude of the P300 component of event-related brain potentials was proposed (Farwell, L. A. & Smith, S. S. J. Forensic Sci. 46, 135-143, 2001). When a human subject is exposed to something that already is stored in memory, the brain emits an electrical response called a P300 wave. The P300 is a non-specific brain response, elicited in response to innovation, subject's own name, surprise and task related stimuli. This phenomenon occurs approximately 300 milliseconds after a meaningful stimulus. The investigators extrapolate from an Electro-Encephalo-Graphy (EEG) recording clues to differentiate between relevant and irrelevant stimuli.
Another important known technique for mind state detection is the functional magnetic resonance imaging (fMRI). Images of the brain can be taken during activity at rest and during a specific behavior, thus demonstrating the function of a particular area of the subject's brain as well as its structure. An extensive research is performed to detect the part of the brain active during a lie. There are a number of reports in the literature about success in detecting deception (Langleben, D. D. et al. Neurolmage 15, 727-732, 2002), (Kozel F. A. et al. Behav Neurosci. 2004 August; 118(4):852-6), (Ganis G et al. Cereb Cortex. 2003 August; 13(8):830-6), a number of brain regions were found to participate in the deceptive response. This is an important discovery, since even though a subject might control his autonomic responses, the very thought of deception will be detected. A few studies were also done concerning the subjective emotional experience of subjects. However, fMRI require a large and expensive and non-mobile instrumentation, and cannot get results within milliseconds resolution.